The invention relates to a novel process for purifying maltose-containing liquors, such as maltose syrups.
Maltose is a valuable raw material in the production of maltitol (xcex1(1xe2x86x924)glucosylsorbitol), which is a sugar alcohol generally used as a sweetening agent in low-caloric, dietary and low-cariogenic foods, such as confectionary products and chewing gums. Maltitol is prepared in the form of crystalline maltitol or maltitol syrup.
Maltose is produced from a starch solution, which is first enzymatically hydrolyzed into a maltose syrup. For the production of maltitol, maltose syrup is catalytically hydrogenated to maltitol, whereafter the maltitol syrup is crystallized. The maltose syrup used as the starting material for the hydrogenation and crystallization contains varying levels of undesirable impurities, especially maltotriose. Maltotriose has a tendency to make the final maltose product unstable and hygroscopic. Furthermore, the presence of maltotriose may disturb the crystallization of maltose and maltitol. For preparing crystalline products of high purity, it is thus necessary to purify the maltose-containing syrup from maltotriose. Various methods, such as hydrolysis with enzymes, chromatography and ultrafiltration or combinations thereof have been used for the purification of maltose syrups.
An enzymatic hydrolysis method for the production of maltose has been disclosed e.g. in U.S. Pat. No. 4,408,041 (Hayashibara). Chromatographic methods for the purification of maltose have been disclosed in U.S. Pat. Nos. 3,817,787 (Suomen Sokeri Oy) and 4,487,198 (Hayashibara), for example.
Ultrafiltration for the purification of liquors containing maltose and glucose have been described e.g. in U.S. Pat. No. 4,429,122 (UOP Inc.). This U.S. Patent discloses a process for the separation of a mono- or disaccharide, such as glucose and/or maltose, from polysaccharides by passing a mixture containing monosaccharides, disaccharides and polysaccharides through an ultrafiltration membrane. Polysaccharides are retained on the ultrafiltration membrane, while monosaccharides and disaccharides are permeated through the membrane. In this process, maltose and/or glucose are separated from oligosaccharides, but not from impurities having a smaller molar mass, such as maltotriose.
U.S. Pat. No. 4,511,654 (UOP Inc.) relates to a process for the production of a high glucose or maltose syrup by treating a glucose/maltose-containing feedstock with an enzyme selected from amyloglucosidase and xcex2-amylase to form a partially hydrolyzed reaction mixture, passing the resultant partially hydrolyzed reaction mixture through an ultrafiltration membrane to form a retentate and a permeate, recycling the retentate to the enzyme treatment stage, and recovering the permeate including the high glucose or maltose syrup. Even in this process, the resulting glucose/maltose syrup is not free from impurities, such as maltotriose.
Japanese Patent Publication JP 51098346 A (Ajinomoto KK) discloses the preparation of high purity maltose by reacting gelatinized starch with xcex2-amylase and ultrafiltering the solution thus obtained using a semipermeable membrane having a cut-off size of 5000 to 50000 g/mol, preferably 10000 to 30000 g/mol. A highly pure maltose is obtained as the filtrate.
Nanofiltration is a relatively new pressure-driven membrane filtration process, falling between reverse osmosis and ultrafiltration. Nanofiltration typically retains large and organic molecules with a molar mass greater than 300 g/mol. The most important nanofiltration membranes are composite membranes made by interfacial polymerisation. Aromatic polyamide membranes, polysulfone membranes, sulfonated polysulfone membranes, polyether sulfone membranes, sulfonated polyether sulfone membranes, polyester membranes and polypiperazine membranes are examples of widely used nanofiltration membranes. Inorganic and ceramic membranes can also be used for nanofiltration.
U.S. Pat. No. 5,869,297 (Archer Daniels Midland Co.) discloses a nanofiltration process for making dextrose. This process comprises nanofiltering a dextrose composition including as impurities higher saccharides, such as disaccharides and trisaccharides. A dextrose composition having a solids content of at least 99% dextrose is obtained. Crosslinked aromatic polyamide membranes have been used as nanofiltration membranes.
WO 99/28490 (Novo Nordisk AS) discloses a method of producing di- and oligosaccharide syrups by enzymatic reaction of saccharides followed by nanofiltration of the enzymatically treated saccharide solution to obtain as the retentate an oligosaccharide syrup containing disaccharides and higher saccharides. A thin film composite polysulfone membrane having a cut-off size less than 100 g/mol has been used as the nanofiltration membrane, for example. In one embodiment of the process, a liquefied starch solution of maltodextrins is used as the starting material for the enzymatic reaction and subsequent nanofiltration.
U.S. Pat. No. 6,126,754 (Roquette Freres) relates to a process for the manufacture of a starch hydrolysate with high dextrose content. In this process, a starch milk is subjected to enzymatic treatment to obtain a raw saccharifed hydrolysate. The hydrolysate thus obtained is then subjected to nanofiltering to collect as the nanofiltration permeate the desired starch hydrolysate with a high dextrose content.
The purpose of the present invention is to provide a method for purifying a maltose-containing liquor from maltotriose using membrane filtration techniques. The process of the claimed invention is based on the use of nanofiltration.
In accordance with the present invention, complicated and cumbersome purification methods, such as chromatographic steps can be completely or partly replaced by less complicated nanofiltration membrane techniques. The process of the present invention can provide a maltose solution essentially free from undesired low molar-mass impurities, such as maltotriose.
The invention relates to a process for purifying a maltose-containing liquor from maltotriose, wherein said maltose-containing liquor has a maltose content of at least about 55% by weight, based on dissolved dry solids, by nanofiltering said liquor and recovering as the permeate a maltose solution having an increased ratio of maltose to maltotriose.
In a typical embodiment of the invention, the process comprises recovering a maltose solution having a ratio of maltose to maltotriose of over 1.1 times, preferably over 5 times, more preferably over 10 times and most preferably over 20 times that of the starting liquor. Typically, the process comprises recovering a maltose solution having a ratio of maltose to maltotriose of 1.1. to 30 times, preferably 5 to 30 times, more preferably 10 to 30 times and most preferably 20 to 30 times that of the starting liquor.
The maltose content of the starting liquor is at least about 55% by weight, preferably at least about 80% by weight, based on dissolved dry solids. The maltose content is typically in the range of 55 to 90%, preferably 80 to 90% by weight, based on dissolved dry solids.
The separation of maltose from maltotriose can be regulated by varying the maltose content of the starting maltose-containing liquor.
The maltose-containing liquor to be treated by the process of the invention may be a maltose syrup, for example.
The dry substance content of the starting maltose-containing liquor is typically 5 to 50% by weight, preferably 8 to 25% by weight.
The maltose-containing liquor used as the starting material usually contains also monosaccharides, mainly glucose, in a typical amount of 10 to 95%, based on the maltose content. The starting liquor may also contain minor amounts of other monosaccharides. Furthermore, the starting maltose-containing liquor typically contains oligosaccharides and small amounts of ionic compounds, such as metal cations, e.g. sodium, potassium, calcium, magnesium and iron cations.
The maltose-containing liquor to be treated is typically obtained from a starch solution, which is typically hydrolyzed into a maltose syrup. The hydrolysis can be carried out with enzymes, for example.
The process of the invention may also comprise one or more pretreatment steps. The pretreatment before the nanofiltration is typically selected from ion exchange, ultrafiltration, chromatography, concentration, pH adjustment, filtration and combinations thereof. Before the nanofiltration, the starting liquor may be thus pretreated by ion exchange, ultrafiltration or chromatography, for example. Furthermore, a prefiltering step to remove the solid substances can be used before the nanofiltration. The pretreatment of the starting liquor may also comprise concentration, e.g. by evaporation. The pretreatment may also comprise crystallization, whereby the starting liquor may also be a mother liquor obtained from the crystallization of maltose.
The nanofiltration is typically carried out at a pH of 1 to 8, preferably 4 to 8, most preferably 4.5 to 7.0. If necessary, the pH of the starting liquor is adjusted to the desired value before nanofiltration.
The nanofiltration is typically carried out at a pressure of 10 to 50 bar, preferably 15 to 35 bar. A typical nanofiltration temperature is 5 to 95xc2x0 C., preferably 30 to 60xc2x0 C. The nanofiltration is typically carried out with a flux of 10 to 100 l/m2h.
The separation of maltotriose from maltose can also be regulated by varying the pressure and temperature of the nanofiltration operation, besides varying the maltose content of the starting liquor mentioned above. As a rule, the higher the temperature and the pressure, the better separation is achieved.
The nanofiltration membrane used in the present invention can be selected from polymeric and inorganic membranes having a cut-off size of 100-2500 g/mol, preferably 500 to 2500 g/mol.
Typical polymeric nanofiltration membranes useful in the present invention include, for example, aromatic polyamide membranes, polysulfone membranes, sulfonated polysulfone membranes, polyether sulfone membranes, sulfonated polyether sulfone membranes, polyester membranes and polypiperazine membranes and combinations thereof. Cellulose acetate membranes are also useful as nanofiltration membranes in the present invention.
Typical inorganic membranes include ZrO2- and Al2O3-membranes, for example.
Preferred nanofiltration membranes are selected from aromatic polyamide/polysulfone membranes and sulfonated polyether sulfone membranes. As specific useful membranes can be mentioned Desal G10 nanofiltration membrane (manufacturer Osmonics) and NTR-7450 nanofiltration membrane (manufacturer Nitto Denko), for example.
The nanofiltration membranes which are useful in the present invention may have a negative or positive charge. The membranes can be ionic membranes, i.e. they may contain cationic or anionic groups, but even neutral membranes are useful. The nanofiltration membranes may be selected from hydrophobic and hydrophilic membranes.
The typical form of nanofiltration membranes is a flat sheet form. The membrane configuration may also be selected e.g. from tubes, spiral membranes and hollow fibers. xe2x80x9cHigh shearxe2x80x9d membranes, such as vibrating membranes and rotating membranes can also be used.
Before the nanofiltration procedure, the nanofiltration membranes may be pretreated with water, alkaline detergents and/or ethanol, for example.
In a typical nanofiltration operation, the liquor to be treated is fed through the nanofiltration membrane using the temperature and pressure conditions described above. The liquor is thus fractionated into a low molar mass fraction including maltose (permeate) and a high molar mass fraction including the non-desired components of the starting maltose-containing liquor (retentate).
The nanofiltration equipment useful in the present invention comprises at least one nanofiltration membrane element dividing the feed into a retentate and permeate section. The nanofiltration equipment typically also include means for controlling the pressure and flow. The equipment may also include several nanofiltration membrane elements in different combinations, arranged in parallel or series.
The flux of the permeate varies in accordance with the pressure. In general, at a normal operation range, the higher the pressure, the higher the flux. The flux also varies with the temperature. An increase of the operating temperature increases the flux. However, with higher temperatures and with higher pressures there is an increased tendency for a membrane rupture. For inorganic membranes, higher temperatures and pressures and higher pH ranges can be used than for polymeric membranes.
The nanofiltration in accordance with the present invention can be carried out batchwise or continuously. The nanofiltration procedure can be repeated once or several times.
After nanofiltration, the maltose may be recovered from the permeate, e.g. by crystallization. The nanofiltered solution can be used as such for the crystallization, without further purification and separation steps. If desired, the nanofiltered maltose solution can be subjected to further purification, e.g. by chromatography, ion exchange, concentration by evaporation or reverse osmosis, or colour removal.
In the process of the present invention, the purified maltose solution obtained as the permeate is also as a rule enriched in glucose and deprived of oligosaccharides.
The process of the invention may comprise a further step of separating the glucose from the permeate. Glucose is typically separated by nanofiltration or chromatography.
The process of the invention may also comprise a further step of recovering a solution enriched in oligosaccharides as the retentate.
The invention also relates to a purified maltose product thus obtained. Furthermore, the invention relates to the use of the maltose product thus obtained for the preparation of maltitol in a crystalline form or in the form of a solution. For preparing maltitol, maltose thus obtained can be used either before or after the separation of glucose. The maltose product obtained by the process of the invention can be used in the form of a maltose solution or in a crystalline form after the crystallization of maltose.
Furthermore, the invention relates to the use of the maltose product obtained according to the process of the present invention for the preparation maltitol by the conversion of maltose to maltitol, for example by catalytic hydrogenation.
The invention also relates to the use of the maltose product obtained by the present invention in foodstuffs. In this embodiment of the invention, maltose is typically used in the form of maltose syrup or maltose crystals.
Preferred embodiments of the invention will be described in greater detail by the following examples, which are not construed as limiting the scope of the invention.
In the examples and throughout the specification and claims, the following definitions have been used:
RDS refers to the refractometric dry substance content, expressed as % by weight.
Flux refers to the amount (liters) of the solution that permeates through the nanofiltration membrane during one hour calculated per one square meter of the membrane surface, l/(m2h).
Retention refers to the proportion of the measured compound retained by the membrane. The higher the retention value, the less is the amount of the compound transferred through the membrane:
Retention (%)=[(Feedxe2x88x92Permeate)/Feed]xc3x97100
where xe2x80x9cFeedxe2x80x9d refers to the concentration of the compound in the feed solution (expressed e.g. in g/l) and xe2x80x9cPermeatexe2x80x9d refers to the concentration of the compound in the permeate solution (expressed e.g. in g/l).
The following membranes were used in the examples:
NTR-7450 (a sulfonated polyethersulfone membrane having a cut-off size of 500 to 1000 g/mol, permeability (25xc2x0 C.) of 9.4 l/(m2h bar), NaCl-retention of 51% (5 g/l), manufacturer Nitto Denko),
Desal G10 (a thin film membrane of aromatic polyamide/polysulfone material having a cut-off-size of 2500 g/mol, permeability (25xc2x0 C.) of 3.4 /l(m2h bar), NaCl-retention of 10%, retention of dextrane (1500 g/ml) of 95%, retention of glucose of 50%, manufacturer Osmonics),
NF 200 (a polypiperazine membrane having a cut-off size of 200 g/mol, permeability (25xc2x0 C.) of 7-8 l/(m2h bar), NaCl-retention of 70%, manufacturer Dow Deutschland),
ASP 10 (a membrane consisting of sulfonated polysulfone on polysulfone, having a permeability (25xc2x0 C.) of 16 l/(m2h bar), NaCl-retention of 10%, manufacturer Advanced Membrane Technology),
TS 40 (a membrane consisting of fully aromatic polyamide, having a permeability of (25xc2x0 C.) of 5.6 l/(m2h bar), manufacturer TriSep),
ASP 20 (a membrane consisting of sulfonated polysulfone on polysulfone, having a permeability (25xc2x0 C.) of 12.5 l/(m2h bar), NaCl-retention of 20%, manufacturer Advanced Membrane Technology),
UF-PES-4H (a membrane consisting of polyethersulfone on polypropylene, having a cut-off size of about 4000 g/mol, a permeability (25xc2x0 C.) of 7 to 17 l/(m2h bar), manufacturer Hoechst),
NF-PES-10 (a polyethersulfone membrane, having a cut-off size of 1000 g/mol, a permeability (25xc2x0 C.) of 5 to 11 l/(m2h bar), NaCl-retention less than 15% (5 g/l), manufacturer Hoechst),
NF45 (a membrane consisting of aromatic polyamide, having a permeability (25xc2x0 C.) of 4.8 l/(m2h bar), NaCl-retention of 45%, manufacturer Dow Deutschland).
Furthermore, the following membranes are useful in the process of the invention:
Desal-5 DK ( a four-layered membrane consisting of a polyester layer, a polysulfone layer and two proprietary layers, having a cut-off size of 150 to 300 g/mol, permeability (25xc2x0 C.) of 5.4 l/(m2h bar) and MgSO4-retention of 98% (2 g/l), manufacturer Osmonics),
Desal-5 DL (a four-layered membrane consisting of a polyester layer, a polysulfone layer and two proprietary layers, having a cut-off size of 150 to 300 g/mol, permeability (25xc2x0 C.) of 7.6 l/(m2h bar), MgSO4-retention of 96% (2 g/l), manufacturer Osmonics),
TFC S (a membrane consisting of modified aromatic polyamide; having a cut-off size of 200 to 300 g/mol, a permeability (25xc2x0 C.) of 7.7 l/(m2h bar), NaCl-retention of 85% (2 g/l), manufacturer Fluid Systems).